1) A benefactor doesn't really exist. It is basically people asking for a rich high level character to fund them through the early stages of the game. This rarely succeeds and when it does it is a result of "knowing" them through chat/playing. 

For long term players it is the use of their diggers/money makers acting as "benefactors" for their other characters by transferring cash and items to them through the market (there is a Market Transfer Guide explaining this).

2) Basically put as a boat costs 5 million a "benefactor" whether another player or your own digger is needed to fund the boat such that you can get to TA.

3) A digger/nail thief can go to TA at any level. For fighters/mages it is really only worthwhile after level 30 (though there have been exceptions with people getting kills at lvl 26-7, this though is rare).

4) There is a TA Players Guide on the forum, furthermore the player's help sites tell you about the guards etc - read it and them.

As an aside, when you have questions please use the search forum feature and look at the help info/guides/player websites before you ask them. If they have been asked before or a solution is readily available I shall no longer answer in detail as it clogs the forum and it does not seem fair for myself and others to put in time and effort in helping, when those asking have been to lazy to help themselves. 

Enjoy Playing - use the info available

KmS